Abstract:The high content of chlorine limits the reuse of municipal solid waste (MSW) fly ash in cement industry and dechlorination is the major technical problem to be solved. Based on the analysis of content and chemical properties of chlorine in the fly ash from 7representative MSW incineration plants in China, the co-disposal process of fly ash and excess sludge was developed to provide theoretical and technical support for the utilization of fly ash. The content of chlorine in the fly ash from representative incineration plants was 4.6%~12.7% (wt); the contents of chlorine in fly ash from different regions were different due to various waste compositions; industrial waste is the main source of insoluble chlorine in fly ash; by comparing the synchrotron XRD analysis of washed and unwashed fly ash, it was found that the speciation of soluble chlorine includes KCl, NaCl and CaClOH and the speciation of insoluble chlorine is AlOCl; the content of chlorine in fly ash decreased to 0.03% (wt) and met Chinese national standard, GB175-2007, after 150d co-disposal of fly ash and excess sludge with the mixing ratio of 8:2; volatile fatty acids produced by fermentation of excess sludge decreased the pH of co-disposal system and converted insoluble chlorine into soluble chlorine to accelerate the dechlorination of fly ash.